Silver halide photothermographic imaging materials, often referred to as `dry silver` compositions because no liquid development is necessary to produce the final image, have been known in the art for many years. These imaging materials basically comprise a light insensitive, reducible silver source, a light sensitive material which generates silver when irradiated, and a reducing agent for the silver source. The light sensitive material is generally photographic silver halide which must be in catalytic proximity to the light insensitive silver source. Catalytic proximity is an intimate physical association of these two materials so that when silver specks or nuclei are generated by the irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the silver source by the reducing agent. It has been long understood that silver is a catalyst for the reduction of silver ions and the silver-generating light sensitive silver halide catalyst progenitor may be placed into catalytic proximity with the silver source in a number of different fashions, such as partial metathesis of the silver source with a halogen-containing source (e.g., U.S. Pat. No. 3,457,075), coprecipitation of the silver halide and silver source material (e.g., U.S. Pat. No. 3,839,049), and any other method which intimately associates the silver halide and the silver source.
The silver source used in this area of technology is a material which contains silver ions. The earliest and still preferred source comprises silver salts of long chain carboxylic acids, usually of from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of like molecular weight have been primarily used. Salts of other organic acids or other organic materials such as silver imidazolates have been proposed, and U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as image source materials.
In both photographic and photothermographic emulsions, exposure of the silver halide to light produces small clusters of silver atoms. The imagewise distribution of these clusters is known in the art as the latent image. This latent image generally is not visible by ordinary means and the light sensitive article must be further processed in order to produce a visual image. The visual image is produced by the catalytic reduction of silver ions which are in catalytic proximity to the specks of the latent image.
As the visible image is produced entirely by silver, one cannot readily reduce the amount of silver in the emulsion without reducing the available maximum image density. Reduction of the amount of silver is desirable in order to reduce the cost of raw materials used in the emulsion.
One traditional way of attempting to increase the image density of photographic and photothermographic emulsions without increasing or while decreasing the amount of silver in the emulsion layer is by the addition of dye forming materials into the emulsion.
U.S. Pat. No. 4,021,240 discloses the use of sulfonamidophenol reducing agents and four equivalent photographic color couplers in thermographic and photothermographic emulsions to produce dye images.
U.S. Pat. No. 4,022,617 discloses the use of leuco dyes (referred to as leuco base dyes) in photothermographic emulsions. These leuco dyes are oxidized to form a color image during the heat development of the photothermographic element. A number of useful toners and development modifiers are also disclosed.
Various color toning agents which modify the color of the silver image of photothermographic emulsions and darken it to a black or blue-black image are also well known in the art as represented by U.S. Pat. Nos. 4,123,282; 3,994,732; 3,846,136 and 4,021,249.
U.S. Pat. No. 3,985,565 discloses the use of phenolic type photographic color couplers in photothermographic emulsions to provide a color image.
U.S. Pat. No. 3,531,286 discloses the use of photographic phenolic or active methylene color couplers in photothermographic emulsions containing p-phenylenediamine developing agents to produce dye images.
Research Disclosure 17029, "Photothermographic Silver Halide Systems," published June 1978, pp. 9-15, gives a brief history of photothermographic systems and discusses attempts to provide color to them. Many of these previously discussed patents and other art such as U.S. Pat. Nos. 4,022,617; 3,180,731 and 3,761,270 are noted as relevant to the subject of providing dye density and color images to photothermographic emulsions.
One problem which has been encountered in the construction of these systems is the traditional problem of balancing the development rate of the emulsion with the shelf-stability of the emulsion. The more rapidly color may be developed in the emulsion during thermal development, the greater tendency the emulsion has to form dyes without exposure and heating. Classically, whatever one does to speed up the rate of color formation tends to increase the formation of spurious dye images (i.e., background coloration). The use of fast coupling color couplers or easily oxidizable leuco dyes in photothermographic systems consistently tends to increase the amount of spurious dye imaging which occurs. This is analogous to fog in photographic emulsions.
It would be desirable to have high speed color image or color enhanced image photothermographic emulsion without loss of shelf stability.